1. Field of the Invention
The invention involves a method for joining a semiconductor integrated circuit chip to a chip carrier substrate and the resulting chip package.
2. Description of the Related Art
In one class of chip packages, one or more semiconductor integrated circuit chips are mounted face down, in the so-called flip chip configuration, onto solderable metal pads on the circuitized surface of a chip carrier substrate, e. g., a ceramic substrate, via solder balls. Such mounting is achieved by applying solder balls, having a composition which includes, for example, 97 percent (by weight) Pb and 3 percent (by weight) Sn, to contact pads on the circuit-bearing surface of each semiconductor chip. In addition, a solder mask is formed on the circuitized surface of the chip carrier substrate to prevent solder from flowing beyond the boundaries of the solderable metal contact pads on this circuitized surface. Then, the solder balls on the chip contact pads are brought into contact with the solderable metal contact pads on the substrate circuitized surface, and a fluxing agent, often including chloro-fluoro-carbons (CFCs), is applied to the solder balls in order to eliminate oxide layers which may have formed on the surfaces of the solder balls and/or solderable metal contact pads. Finally, the solder balls are heated to a sufficiently high temperature, e.g., 330 degrees C., so that they melt and undergo reflow. Upon cooling and re-solidification, the solder balls form firmly adherent, mechanical and electrical connections between the contact pads on each chip and the solderable metal contact pads on the chip carrier substrate.
If the chip carrier substrate is, for example, a ceramic substrate, such as an alumina substrate, then the above-described chip joining process is definitely effective and results in a useful and commercially valuable chip package. That is, neither the presence of the solder mask, nor the reflow process, during the chip joining process is at all adverse to the final chip package. Of course, the use of fluxing agents containing CFCs is considered environmentally undesirable, and attempts are now being made to replace such fluxing agents with fluxing agents which do not contain CFCs.
Significantly, a new type of chip package is now being developed which includes one or more semiconductor chips mounted in the flip chip configuration on an organic chip carrier substrate, such as a polyimide chip carrier substrate. Because such a polyimide substrate is often processed while in a tape- or roll-like form, and because the bonding of a chip or chips to such a polyimide substrate is typically achieved via an automated bonding process, such an organic substrate is often referred to as a TAB (tape automated bonding) substrate or TAB-like substrate. Here, the one or more semiconductor chips are mounted face down, via solder balls, onto solderable metal contact pads or leads or circuit lines (hereinafter generically referred to as solderable metal contact pads) on the circuitized surface of the organic chip carrier substrate.
Unfortunately, the chip joining process used in connection with ceramic chip carrier substrates is disadvantageous when used in connection with organic chip carrier substrates, such as polyimide chip carrier substrates. That is, one of the advantages of, for example, polyimide substrates is that they are highly flexible, which makes them more useful than the relatively rigid ceramic substrates for a variety of applications. However, the presence of a solder mask on a polyimide substrate substantially reduces the flexibility of the substrate, which is obviously undesirable. In addition, the relatively high temperatures needed to melt and reflow solder balls distorts polyimide substrates and thereby subtantially degrades the dimensional stability of such substrates, which makes subsequent processing of such substrates very difficult, if not impossible. Moreover, these reflow temperatures are so high that polyimide substrates are often on the verge of decomposing, while other chip carrier substrates, such as epoxy/glass chip carrier substrates, do decompose at these temperatures.
Thus, those engaged in the development of chip packages which include organic chip carrier substrates have sought, thus far with relatively little success, methods for joining chips to organic chip carrier substrates which do not require the use of solder masks, which do not require the melting of solder balls and which do not require the use of fluxing agents.